1. Field of the Invention
The present invention relates to a microspeaker with an inner resonance chamber, and more particularly to a microspeaker with an inner resonance chamber which improves quality of sound and enables slim and compact design of the microspeaker by blocking rearward sound generated at the read side of the vibration plate to prevent interference of a rearward sound with a forward sound generated at the front side of a vibration plate and installing a chamber with a specific volume within the microspeaker to allow the rearward sound to cause resonance.
2. Description of the Related Art
In general, portable electronic devices including portable communication terminals, laptop computers, and MP3 players are provided with a microspeaker for converting an electric signal into an acoustic signal. Such a microspeaker converts electrical energy into mechanical energy by positioning a voice coil in an air gap of the magnetic circuit, according to Fleming's left-hand rule, which states that when a coil of wire carrying an electric current is placed in a magnetic field, the coil is caused to move.
The microspeaker includes a magnet, an upper plate, and a yoke member to configure a magnetic circuit, and a voice coil is installed in the air gap to interlink with the magnetic flux of the magnetic circuit. The voice coil is adhered to the lower surface of a vibration plate which is adhesively bound at its edges by a frame, so that and attractive and repulsive forces may be generated between an electromotive force generated by an input signal applied to the voice coil and the magnetic circuit to cause the vibration plate to vibrate to generate sounds.
As the vibration plate vibrates back and forth as above, forward sound is generated at the front side of the vibration plate, while rearward sound is generated at the rear side of the vibration plate. The forward sound generated at the front side of the vibration plate is radiated forward through a forward sound emitting outlet provided at the front side of the microspeaker, and the rearward sound generated at the rear side of the vibration plate is discharged rearward through a rearward sound emitting outlet provided at the rear side of the microspeaker. The present invention is directed to processing rearward sound generated at the rear side of the vibration plate. That is, since the forward sound radiated forward from the front side of the vibration plate has a phase difference of 180° from the rearward sound radiated rearward from the rear side of the vibration plate, interference may occur when the forward sound meets the rearward sound in a narrow space, resulting in degradation of quality of low frequency sounds. Therefore, if the rearward sound generated at the rear side of the vibration plate is not blocked, low frequency sounds undergo destructive interference, and only high frequency sounds remain, thereby making it difficult to reproduce natural sounds.
Accordingly, the present invention is directed to blocking rearward sounds generated at the rear side of the vibration plate from interfering with forward sounds generated at the front side of the vibration plate. Conventionally, an enclosure has been used to block the rearward sound generated at the rear side of the vibration plate. The enclosure accommodates the microspeaker to serve to block the rearward sound generated at the rear side of the vibration plate. Here, if the rearward sound generated at the rear side of the vibration plate is completely blocked, air pressure at the rear side of the vibration plate rises to cause a narrow amplitude and shift of the sound to a higher frequency range, leading to degradation of sound quality. To address this problem, a chamber (back volume) having a certain size has conventionally been installed in the enclosure to damp the rearward sound generated at the rear side of the vibration plate to improve the characteristics of low frequency sounds. In improving the characteristics of the low frequency range, a back volume having a large size is more advantageous. Thus, in the conventional cases, a back volume having an adequate size has been formed between the microspeaker and the enclosure. However, as the size of the enclosure increases due to the back volume, there is a problem of causing the microspeaker module to be thicker. In the field of the present invention in which slim and compact designs are pursued, a thinner microspeaker is advantageous. Accordingly, there is a need to limit the volumetric size of the back volume to reduce the thickness of the microspeaker module.
Meanwhile, for a microspeaker used for an in-ear earphone, interference between the forward sound and the rearward sound rarely occurs, but if a rearward sound is not blocked, a shrill sound (a hissing sound) is caused by a resonance occurring in the outer ear, and the sound is shifted to the high frequency range, and thereby there is a problem of degradation of the sound quality. Therefore, there is a need to improve the quality of sound through adjustment to an even balance in the high frequency range by increasing sound pressure to allow the rearward sound generated at the rear side of the vibration plate to cause resonance and at the same time to shift the resonant frequency in the high frequency range to a lower frequency range.
For example, as shown in FIG. 15, a conventional speaker module 220 (Korean Patent Application Publication No. 10-2007-0021014) defines a back volume S20 of a predetermined size at the rear side of the spear unit 224 to block the rearward sound to improve the characteristics of the low frequency sounds. However, in case of the conventional back volume S20, a cable 226 installed separately at the outside of a speaker unit 224 and connected to the speaker unit 224 is positioned in the back volume S20. In addition, solder is formed in the back volume S20 to fix the cable 226 to a connection terminal of the speaker unit 224.
Thereby, the space of the conventional back volume S20 is reduced by the cable 226 and the solder. In particular, for a subminiature microspeaker like an in-ear earphone, a space available for installation of the back volume is limited, and therefore reduction of the space due to the cable and the solder has a considerable effect on the microspeaker. Further, for the conventional microspeakers, since the sizes of the cable and the solder located in the back volume are uncontrollable, the back volume varies from one product to another. Accordingly, it is difficult to make the rearward sound cause resonance with a conventional microspeaker. That is, for the rearward sound to cause resonance, the natural frequency of the vibration plate should coincide with that of the back volume. But in conventional microspeakers, the size of the back volume cannot be precisely controlled, and thus acoustic resonance cannot be caused. Accordingly, the back volume of a conventional microspeaker, which has been used to improve characteristics of low frequency sounds, has a problem of failing to effectively control the shrill sound in the high frequency range.